Which is faster for syncing iPhone 4 with iTunes -- USB or WIFI?
Is USB or WIFI faster when syncing an iPhone 4 with iOS 5 to iTunes?
This SuperUser answer suggests that Wireless N might be faster than USB 2: https://superuser.com/questions/288705/speed-comparison-usb-vs-wireless-n-vs-cat-6
Note: iPhone 4 is 802.11b/g/n Wi-Fi (802.11n 2.4GHz only)
USB 2.0 is faster, but Wi-Fi is better.
In practice, Wi-Fi will be superior to USB for syncing and backup since you don't have to worry about plugging it in. It can start as soon as you enter the network, throttle itself based on CPU/iOS activity. You can even start on WiFi and connect your iOS 5 device to USB and syncing will continue at faster speeds if WiFi proves to be slow for a particular sync session.
My experience is the devices can do their thing and sync in the background without me waiting for them and I'm almost always the slowest peg in the process. :-)
However, USB 2.0 speed is faster than 802.11n both in theory and in practice. See this chart from a very nice Apple Insider article on relative speeds relating to Time Capsule performance. The one area where USB (or any other wired connection whether it's USB 2.0, 3.0, FireWire or Thunderbolt) is better than Wi-Fi is latency. If your sync session is largely lots of little checks and no appreciable time is spent waiting for large files to transfer, then the wired connection could be much faster if your sessions are shorter than 15 to 30 seconds in length. Here's another nice article explaining how even a slow USB 2.0 connection can be fast enough for some demanding data transfers: http://www.apogeedigital.com/knowledgebase/quartet/why-doesnt-quartet-use-usb-3-0/
AppleInsider: Exploring Time Capsule: theoretical speed vs practical throughput
In my experience, syncing at a variety of times, with an 802.11n network, USB is approximately 4x faster than WiFi.
If you're in the middle of a WiFi sync and it's taking too long, you can connect the USB cable and the sync will complete over the (much) faster USB connection.